posted on 2012-09-11, 00:00authored byNavneet Dogra, Xuelian Li, Punit Kohli
We investigate interactions between receptors and ligands
at bilayer
surface of polydiacetylene (PDA) liposomal nanoparticles using changes
in electronic absorption spectroscopy and fluorescence resonance energy
transfer (FRET). We study the effect of mode of linkage (covalent
versus noncovalent) between the receptor and liposome bilayer. We
also examine the effect of size-dependent interactions between liposome
and analyte through electronic absorption and FRET responses. Glucose
(receptor) molecules were either covalently or noncovalently attached
at the bilayer of nanoparticles, and they provided selectivity for
molecular interactions between glucose and glycoprotein ligands of E. coli. These interactions induced stress on conjugated
PDA chain which resulted in changes (blue to red) in the absorption
spectrum of PDA. The changes in electronic absorbance also led to
changes in FRET efficiency between conjugated PDA chains (acceptor)
and fluorophores (Sulphorhodamine-101) (donor) attached to the bilayer
surface. Interestingly, we did not find significant differences in
UV–vis and FRET responses for covalently and noncovalently
bound glucose to liposomes following their interactions with E. coli. We attributed these results to close proximity
of glucose receptor molecules to the liposome bilayer surface such
that induced stress were similar in both the cases. We also found
that PDA emission from direct excitation mechanism was ∼2–10
times larger than that of the FRET-based response. These differences
in emission signals were attributed to three major reasons: nonspecific
interactions between E. coli and liposomes, size
differences between analyte and liposomes, and a much higher PDA concentration
with respect to sulforhodamine (SR-101). We have proposed a model
to explain our experimental observations. Our fundamental studies
reported here will help in enhancing our knowledge regarding interactions
involved between soft particles at molecular levels.